DC is more problematic to switch than AC and a switch or relay designed for
AC must be derated (often massively) to use with DC.

But what about the reverse?

I have an automotive type relay rated 40A at 14V.

That should handle 10A (actually less) at 120VAC okay, yes?

Consequence of either failure to open or failure to close is near zero
(maybe a little wasted electricity). But I don't want it burning up
either. I would think something with a 40A DC rating is, if anything,
overkill. But the voltage is higher. However, I cannot imagine that the
contacts are left close enough together that the 120V (170Vpp) is an issue
as they'd be more likely to draw out and sustain an arc when used on the DC
it was designed for. So I think I am good.

On 9/08/2016 9:58 AM, Chet Kincaid wrote:
DC is more problematic to switch than AC and a switch or relay designed for
AC must be derated (often massively) to use with DC.

But what about the reverse?

I have an automotive type relay rated 40A at 14V.

That should handle 10A (actually less) at 120VAC okay, yes?

Consequence of either failure to open or failure to close is near zero
(maybe a little wasted electricity). But I don't want it burning up
either. I would think something with a 40A DC rating is, if anything,
overkill. But the voltage is higher. However, I cannot imagine that the
contacts are left close enough together that the 120V (170Vpp) is an issue
as they'd be more likely to draw out and sustain an arc when used on the DC
it was designed for. So I think I am good.


**In that case, it is all about insulation quality. Google the relay on
line and see what the manufacturer rates it for on AC. Personally, I
would not muck about with a relay which is not rated for mains. Relays
are cheap. House fires, not so much.

--
Trevor Wilson
www.rageaudio.com.au

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Chet Kincaid wrote:

DC is more problematic to switch than AC and a switch or relay designed for
AC must be derated (often massively) to use with DC.

But what about the reverse?

I have an automotive type relay rated 40A at 14V.

That should handle 10A (actually less) at 120VAC okay, yes?


** As far as the contacts are concerned, you are probably correct.

But relays have another job which is to safely isolate the coil connections from the switched circuit.

There is no guarantee that a relay specified for use in an automotive, 12V DC environment is suitably insulated for connection to the AC supply - where spike voltages in the thousands occur.


..... Phil

"There is no guarantee that a relay specified for use in an automotive, 12V DC environment is suitably insulated for connection to the AC supply - where spike voltages in the thousands occur. "

People who do not know that should not be able to get a screwdriver license or a permit for pliers. Scissors I am not so sure. If they have them they can grab their Darwin award but it is pretty hard to hurt anyone else. So I guess let them have them.

On 08/09/2016 10:17 PM, wrote:
"There is no guarantee that a relay specified for use in an automotive, 12V DC environment is suitably insulated for connection to the AC supply - where spike voltages in the thousands occur. "

People who do not know that should not be able to get a screwdriver license or a permit for pliers. Scissors I am not so sure. If they have them they can grab their Darwin award but it is pretty hard to hurt anyone else. So I guess let them have them.


Do you mean the specs that are printed on parts - like relays - should
be respected?

What an odd idea!

John ;-#)#

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"Old pinballers never die, they just flip out."

"Do you mean the specs that are printed on parts - like relays - should
be respected?

What an odd idea! "

I am old and have funny notions like that.

But I do have to admit to putting foil on the fuses in my amp so it could drive 2.3 ohm loads. One eight ohm and one four ohm per channel in parallel. Actually, never blew the outputs, it didn't even get that hot. Sansui 771.

Phil Allison wrote in
There is no guarantee that a relay specified for use in an automotive,
12V DC environment is suitably insulated for connection to the AC
supply - where spike voltages in the thousands occur.

An excellent point however in this instance the controlling circuit was to
be another 120V circuit with the only reason for 12V being involved at all
was the availability of the relay. I would have just used some old 12V
charger from the junk box. So if hypothetically some big spike had come
down the switched line, jumped over to the coil circuit, and blown through
the P/S it would just have ended up on another 120V circuit which is just
as prepared or not prepared for that as any other. Likewise spikes coming
the other way.

However, it's a moot issue anyway as I realized I need NC contacts while
the 12V relay is SPST NO. Most likely I will dig up a DPDT or 4PDT and
parallel the poles for added ampacity. My "wtf was I thinking" is all
about forgetting I need NC not about why I thought the 12V relay might be
suitable.